The basic operation and structure of land mobile radio communication systems are known. Such radio communication systems typically comprise a plurality of communication units (vehicle mounted or portable radios in a land mobile system and radio/telephones in a cellular system), a predetermined number of transceivers, which are located throughout a geographic region and transceive information via communication channels, and a controlling entity. The controlling entity may either be a centralized call processing controller or it may be a network of distributed controllers working together to establish communication paths for the communication units. The communication channels may be time division multiplex (TDM) slots, carrier frequencies, pairs of carrier frequencies or other radio frequency (RF) transmission mediums. A frequency or time portion of one or more of the communication channels may be established for call control purposes such that a communication unit may communicate with the system controller to request and receive system resources.
Multiple site communication systems which comprise a plurality of repeaters and transceivers that are distributed throughout a large geographic region are also known. Many multi-site systems use same-frequency simulcast, i.e. the same communication channel (or carrier frequency) is used by multiple sites throughout the region to simultaneously relay communications to communication units that are located throughout the multi-site system.
A typical transceiver in a simulcast multi-site communication system comprises an individual circuit that couples the repeater to a central radio system audio collection and signal distribution point (prime site). (Note that any site in the multi-site system may contain a transceiver, i.e. transmitter and receiver, or only a receiver.) Signals, such as control channel or traffic signaling data from the call processing controller, are distributed from the prime site on links to the transceiver sites for simultaneous transmission. Other signals such as, user data from a host computer, or voice dispatch speech from a console must also be distributed from the prime site. To accurately transmit these signals, dedicated, stable, and time-invariant links are used. A dedicated, that is, non-switched, link is used to carry the information to be transmitted from the prime site to each remote site, thus forming a "star" topology network, where the prime site is the center of the star. For example, the links may be analog and/or digital microwave channels. (Note that digital switching networks, for example those provided by public switched telephone network (PSTN) operators, are not used as links because they are not time-invariant.)
With the dedicated, stable, and time invariant links, the site transmitters can broadcast signals in phase, in time, and on the same frequency such that received signal distortion in overlapping site coverage areas is minimal. The stability of the links ensure that the resulting simulcasted signals remain within acceptable tolerances.
To account for the difference in the physical link transport time delays between the prime site and remote site transmitters, additional adjustable delay circuits are typically added to the links. The adjustable delay circuits compensate for the differences in physical link delay such that the total delay is the same at each transceiver site. This ensures that the signal for transmission arrives at each transceiver site at the exact same time. The adjustable time delay devices added to the transmission distribution links may be at the prime or remote sites.
To accommodate for fluctuations in physical link delays, circuits have been devised to manually or automatically adjust the adjustable time delay circuits. Typically, the channel must be excluded from service while a closed loop test is performed to measure and adjust the delay.
Many users of a simulcast system need immediate and constant access to their system channels. For these users, disabling a channel to conduct a closed loop test is inconvenient at best and potentially catastrophic. Such is certainly the case for Public Safety users and centralized controller systems. In a centralized controller system, if the centralized controller is cut off from the system due to a channel being down, communication units cannot communicate. To avoid this, some systems include duplicate prime site equipment, which involves added logic and switching functions that slows the switch-over process.
Other techniques are known which allow the use of time-variant delay links, such as those provided by public digital switching networks. Often these types of links are provided with lower tariffs than those that are time-invariant, making them more attractive for use in simulcast communication systems. However, links are often re-routed in these networks due to traffic overload or when failures occur. The new route may take a completely different path through different links and switches, even through Earth orbit satellites, and thus have a significantly different delay which distorts the simulcast transmission.
Therefore, a need exists for a multi-site simulcast communication system that can efficiently utilize time-invariant or time-variant distribution links, and automatically choose transmitter launch times without the requirement of a dedicated prime site, for signals sourced from any site in the system on a per call basis.